This invention relates to the use of a class of corrosion inhibitors that contain phosphate groups.
Pure metals and their alloys tend to combine with the elements of a corrosive medium to form stable compounds. These compounds are referred to as the corrosion product and the metal surface is said to be corroded.
Corrosion can take several forms. While corrosion is usually confined to the metal surface, it can also occur along grain boundaries because of differences in chemical resistance or local electrolytic action.
In most aqueous systems, the corrosion reaction has anodic and cathodic portions which occur simultaneously at discrete points on the surface of the metal. Electrons flow from an anodic point to a cathodic point to equalize the potential.
In many applications, it may not be economical to use the most corrosion resistant material or a material with satisfactory corrosion resistance may not be known. Other methods of avoiding corrosion, such as the use of glass, ceramic, and organic coatings, may also be prohibitively expensive or incompatible with other process conditions. Similarly, the use of sacrificial anodes to achieve cathodic protection may not achieve satisfactory results or be otherwise practical in many applications. In these cases, an alternative approach is to minimize the corrosion by adding inhibitors to the corrosive medium.
The use of corrosion inhibitors is generally most attractive in closed or recirculating systems in which the annual cost of inhibitor is low. However, inhibitors have also proven to be economical in many once-through systems, such as those encountered in petroleum-processing operations. Inhibitors are effective due to their controlling influence on the cathode or anode area reactions.
Examples of inhibitors used for minimizing corrosion of iron and steel in aqueous solution are the soluble chromates and silicates. Organic sulfide and amine materials are frequently effective in minimizing corrosion of iron and steel in acid solutions. Such inhibitors may also be effective for use with stainless steel and other alloy steel materials.
One disadvantage of many known inhibitors is that these inhibitors are only soluble in the aqueous phase. However, metals are subject to corrosion in oil and gas pipelines, transmission lines, and wells, as well as aqueous systems such as water-disposal systems and processing equipment such as separators, water treatment and dehydration units. This equipment is subject to damaging corrosion which can require expensive replacement or cause catastrophic failure. An extensive amount of time and money has been spent attempting to minimize or eliminate corrosion in a wide variety of materials and environments.
The present invention comprehends a specific type of phosphate ester that is considerably more effective than corrosion inhibitors available in the past. In accordance with the invention, it has now been found that these compounds are highly effective alone or when blended with other specific compounds for protection of ferrous and nonferrous metals against attack by a corrosive environment. An important application of this invention is the prevention of metal corrosion in petroleum and chemical processing equipment, including pipelines, wells, and water treatment units.